There's been a lot of focus on the impact that UKIP could have on the Tory vote at the next election but it's not only David Cameron who needs to worry about his core supporters defecting. In the London Mayoral elections the Lib Dem Brian Paddick came fourth – one spot behind the Green Party's Jenny Jones. The Greens see the next election as a big opportunity to build on the breakthrough that Caroline Lucas made in 2010 when she became her party's first MP.

Last night Ms Lucas announced that she would be stepping down as Green leader (they never used to have one top public spokesperson but a shared, rotating leadership group) in order for a successor to be able to focus on ousting Lib Dems at the next election.

"People who voted Lib Dem in the past feel betrayed… A lot of them are looking for a new home and we want to be able to welcome them… I want to ensure that we use the leadership of the Green Party in a strategic way, to help us build momentum and build up our electoral presence… We don't want to be complacent about it, to assume we have any right to that (Lib Dem) vote but we do want to be able to take our message to Lib Dem areas in particular, to make the case that so many of these issues which we know they care about, the kind of manifesto that they thought they were voting for, are key principles of the Green Party."

The rising profile of the Greens should also worry Ed Miliband. Ms Lucas' party is more red than green – supporting a large state, high taxes on business, a big expansion of regulation and opposition to strong defence. If they start to enjoy a higher profile the Greens could eat into Labour's vote as well as that of the Liberal Democrats.